Data communications in transportation safety systems require high reliability and low latency. For example, the International Electronic Commission (IEC) has set stringent safety and reliability requirements on communication networks in elevator systems. Only one error is allowed in approximately 1015 safety related packets. The latency requirement for high priority packets can be as short as a few milliseconds.
Conventional safety systems are typically implemented with nodes (transceivers) connected to a dedicated wired communication networks. For example, to send safety packets between a controller and a car in an elevator system, a heavy communication cable in an elevator shaft is connected to a moveable car.
Recently, wireless communication technologies have been applied to safety systems to reduce cost and increase scalability. Communication Based Train Control (CBTC) is an example. The communication network in safety systems usually includes multiple fixed nodes such as trackside nodes for CBTC systems, and multiple mobile nodes arranged in train cars. The fixed nodes are connected by a wired network such as Ethernet. Fixed nodes are also capable of transmitting and receiving (transceiving) data wirelessly.
A controller for the safety system is typically connected to at least one fixed node via a wired interface. Packets are transmitted from the controller to a fixed node via the wired interface, and relayed hop-by-hop to all other fixed nodes via the wired network. Then, the fixed nodes retransmit the packet to the mobile nodes using the wireless network. Mobile nodes communicate packets via the wireless network to the fixed nodes. Fixed nodes receive the data, and then relay the data to the fixed node connected to the controller via the wired network. The fixed node connected to the controller then transmits data to the controller via the wired interface.
However, the specifications of existing CBTC systems are insufficient in some aspects. The latency is in the order of seconds due to the use of a conventional Carrier Sense Multiple Access (CSMA) for the wireless network, and the handover process at mobile nodes. Additionally, message error rates can be as high as 10−8.
Therefore, it is desired to develop a communication network for safety systems that achieves higher reliability, such as a message error rate of 10−15, and a latency of a few milliseconds.